The present invention relates to radar systems, particularly to Synthetic Aperture Radar (SAR) systems, and more particularly to a method and apparatus for improving the performance of SAR systems by (a) reducing the effect of "edge losses" associated with nonuniform receiver antenna gain, and (b) reducing "range ambiguity" (undesired radar returns from pulses preceding and following the pulse producing the desired return).
Over the years various radar systems have been developed for different applications. One such radar system now under development at the Lawrence Livermore National Laboratory (LLNL) is the satellite-based SAR system being developed primarily for military, strategic, or earth/environmental resource applications. The SAR system being designed at LLNL is being developed as part of the Tactical Imaging Constellation Architecture Study SAR (CAS/SAR). The TICAS/SAR design requirements included collecting high resolution SAR imagery over a wide target area, with specified signal-to-noise ratio and "multiplicative noise ratio" or MNR. Among the factors that must be included in such a design are "edge losses," generally defined as the reduction in performance at the edge of the useful portion of the radar beam compared to the performance at the center of the beam. Edge losses occur on both transmission and reception. It is possible to make edge losses arbitrarily small by creating a "top hat" antenna pattern with uniform transmit gain and/or receive sensitivity over a range of angles, but this makes very inefficient use of the antenna area. Conventional design practice generally results in edge losses of approximately -3 db (a factor of two in power) in transmit and the same in receive, or -6 dB (a factor of four) two-way. Thus, in order to meet specifications at the edge of the radar beam, up to four times as much power must be transmitted as would be needed to meet specifications at the center of the beam. Edge losses occur in both azimuth (along-track) and elevation (cross-track) directions.
Another key factor in the design of a SAR is the strength of ambiguous returns (the "ambiguity level"), which contributes to and often determines the MNR and the quality of the SAR image. Ambiguous returns result from detecting undesired signals which cannot be distinguished from desired signals. Range-ambiguous signals come from pulses occurring immediately before or after the desired pulse. When the desired signal is being received from the target at range R, the ambiguous signals come from ranges R.+-.c/(2*PRF), where PRF is the Pulse Repetition Frequency. Because the SAR beam is not in the plane of the target but intersects it at an angle (the "grazing angle"), these ambiguous returns are displaced in elevation from the desired return and are suppressed by the directionality of the transmitting and receiving antennas. In some types of SARs (including the LLNL TICAS design) the transmitter must illuminate an area substantially larger than the receiver antenna beam width; in this case, the range-ambiguous responses are attenuated only by the receive antenna pattern.
Reducing or eliminating edge losses in the elevation direction, even in the receive mode only, can reduce the required SAR power by up to 3 dB. Reducing range ambiguities allows either improved image quality or a higher PRF.
The present invention provides a method and apparatus for improving the performance of the SAR system by reducing the effect of "edge losses" associated with nonuniform receiver antenna gain. By moving the receiver antenna pattern, such as by electronic steering, in synchrony with the apparent motion of the transmitted pulse along the ground, the maximum available receiver antenna gain can be used at all times. If the receiver antenna pattern is moved in synchrony with the apparent motion of the preceding and following pulses, the receiver antenna gain for ambiguous signals can be minimized. Both of these benefits are realized simultaneously, although optimizing the beam position for maximum gain may not yield the maximum reaction in ambiguity, and vice versa. The moving or steering of the receiver antenna pattern can be either continuous or in the form of two or more discrete steps. The steering can be implemented in several ways.